[WebAssembly] Fix operand rewriting in inline asm lowering.

Use MachineOperand::ChangeToImmediate rather than reassigning
MachineOperands to new values created from MachineOperand::CreateImm,
so that their parent pointers are preserved.

This fixes "Instruction has operand with wrong parent set" errors
reported by the MachineVerifier.

llvm-svn: 341389